# std::pmr::null_memory_resource

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< cpp‎ | memory

Defined in header `<memory_resource>` |  |
std::pmr::memory_resource* null_memory_resource() noexcept; |  | (since C++17)

Returns a pointer to a `memory_resource` that doesn't perform any allocation.

### Return value

Returns a pointer `p` to a static storage duration object of a type derived from std::pmr::memory_resource, with the following properties:

- its `allocate()` function always throws std::bad_alloc;
- its `deallocate()` function has no effect;
- for any `memory_resource` `r`, p->is_equal(r) returns &r == p.

The same value is returned every time this function is called.

### Example

The program demos the main usage of `null_memory_resource`: ensure that a memory pool which requires memory allocated on the stack will NOT allocate memory on the heap if it needs more memory.

Run this code

    #include <array>
    #include <cstddef>
    #include <iostream>
    #include <memory_resource>
    #include <string>
    #include <unordered_map>

    int main()
    {
        // allocate memory on the stack
        std::array<std::byte, 20000> buf;

        // without fallback memory allocation on heap
        std::pmr::monotonic_buffer_resource pool{buf.data(), buf.size(),
                                                 std::pmr::null_memory_resource()};

        // allocate too much memory
        std::pmr::unordered_map<long, std::pmr::string> coll{&pool};
        try
        {
            for (std::size_t i = 0; i < buf.size(); ++i)
            {
                coll.emplace(i, "just a string with number " + std::to_string(i));

                if (i && i % 50 == 0)
                    std::clog << "size: " << i << "...\n";
            }
        }
        catch (const std::bad_alloc& e)
        {
            std::cerr << e.what() << '\n';
        }

        std::cout << "size: " << coll.size() << '\n';
    }

Possible output:

    size: 50...
    size: 100...
    size: 150...
    std::bad_alloc
    size: 183

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